The present invention is directed to a digital multiplexer system and more specifically, to a system for multiplexing a plurality of PCM data channels into a serial bit stream for transmission to a demultiplexing receiver. The foregoing system in more specific terms is utilized in a microwave transmission system in a second level multiplexer terminal.
In addition, the present invention is directed to a transmission system of the above type in which the digital data scrambler is provided which as is well known in the art eliminates high frequency periodic sequences or creates a more even distribution of power throughout the baseband.
Thus far two types of scramblers have been used; nonsynchronizing and self-synchronizing. Both of these techniques suffer from either loss of channel capacity or excessive errors when a long sequence scrambler is used for sufficient scrambling.
In addition, in a digital data system of the present type which because of its second level nature inherently carries a large amount of information, specifically 19.8 megabits per second, it is important that each frame be maintained in synchronization as continuously as possible. In other words, the framing should be relatively secure even with noise. In addition if loss of frame synchronization occurs, there should be provided relatively fast reframing since the time out-of-frame synchronization is in essence loss of channel capacity.
At the present time, framing is usually accomplished by seeking a simple match with the framing word at the beginning of each frame or with a framing bit. This has been found to be inadequate for high capacity systems.
Systems which provide one long framing word per frame interrupt the data at a certain rate which causes jitter in the demultiplexed data or the reconstituted clock train; the lower the rate the more difficult it is to filter out.